1. Field of the Invention
The invention relates to a signal processing system comprising an output circuit that is tuned to a first frequency, an output of which circuit being connected to an input of a two-port having non-linear amplification, and a frequency-selective external network, an output of the two-port being connected to an input of an input circuit that is tuned to a second frequency.
The invention further relates to a communications system arranged for processing signals in different frequency channels with a predetermined channel spacing, comprising, in succession, an output circuit that is tuned to a channel frequency an output of which being connected to an input of a two-port having non-linear amplification, and a frequency-selective external network, an output of the two-port being connected to an input of an input circuit that is tuned to a second frequency.
Systems of this type are known from the AMPS standard for mobile telecommunication, the two-port having the form of an output amplifier. With the known arrangements it is not very well possible to satisfy the requirements as to reception sensitivity made by the network managers when there is an external interference signal.
If a first and a second signal are available on a two-port that has non-linear amplification, intermodulation signals will arise due to this non-linear amplification. These intermodulation signals appear on the output of the two-port. If the spacing between the first frequency and the second frequency is equal to the spacing between the transmitting frequency of the system and the receiving frequency of the system, an intermodulation component will arise that is exactly equal to the receiving frequency. As a result, the signal-to-noise ratio of the received signal is reduced.
If either of the two signals available on the input of the two-port arrives via an internal feedback from the output to the input, a similar effect will arise. This effect is called Backward Intermodulation.
2. Description of the Related Art
A method of reducing specific components of intermodulation distortion is known from WO 96/25791 Moazzan et al. When two input signals available on the input of a two-port and, having a frequency F1 and F2 respectively, are amplified by a non-linear amplifier, an unwanted third-order IM product having frequency 2*F1xe2x88x92F2 will arise. The signals and IM products will further be referenced by their frequencies. By mixing the second-order distortion component 2*F1 of either of the input signals with the other input signal F2, a new mixed product will arise having a frequency equal to 2*F1xe2x88x92F2. This mixed product 2*F1xe2x88x92F2 is added to the originally unwanted third-order IM product and, with suitable amplitude and phase, compensates for the unwanted third-order IM product. A disadvantage of this method is that both signals must be present on the input and that the second-order distortion component has a frequency that is twice as high as the system frequency, so that the correct amplitude and phase of the correction signal are hard to control. Furthermore, the transistor and associated adaptation networks are also to work at a frequency that is twice as high as the system frequency.
With an input signal F1 having a frequency of, for example, 1800 MHz, and an input signal F2 having a frequency of, for example, 1755 MHz, a third-order IM product will arise at 2*1800xe2x88x921755=1845 MHz. The second harmonic distortion component lies at 2*1800=3600 MHz. If the frequency of input signal F1 of 1755 MHz is mixed with this, a mixed product will arise having a frequency of 3600xe2x88x921755=1845 MHz. This frequency is equal to the frequency of the third-order IM product. With anti-phase and equal amplitude relative to the unwanted third-order IM product, a reduction of this third-order IM product is possible.
It is an object of the invention to minimize the influence of the external interference signal on the receiving sensitivity by compensating a third-order IM product developed in the two-port in a manner that does not have the above identified disadvantages.
For this purpose, the system according to the invention is characterized in that the external network is tuned to a difference frequency between the first frequency and the second frequency and is connected at least to the input of the two-port and to the output of the two-port.
The invention is based on the following recognition:
Besides the unwanted third-order IM product 2*F1xe2x88x92F2, from a first input signal having frequency F1 and a second input signal having frequency F2, with intermodulation as a result of the non-linear amplification of the two-port, also a second-order IM product F1xe2x88x92F2 rises having a relatively low frequency. After this second-order IM product has been fed back and mixed with the input signal F1, inter alia the signal F1+(F1xe2x88x92F2) arises which, provided that a suitable choice of phase and amplitude is made, is capable of reducing the unwanted third-order IM product 2*F1xe2x88x92F2. As a result of the relatively low frequency of F1xe2x88x92F2, the phase and amplitude control becomes considerably simpler.
With a frequency of the first input signal F1 of, for example, 1800 MHz and of the second input signal F2 of, for example, 1755 MHz, a third-order IM product arises at 2*1800xe2x88x921755=1845 MHz. If the receiving frequency is also 1845 MHz, the receiving sensitivity of the signal processing system is reduced. A second-order IM product F1xe2x88x92F2 lies at 1800xe2x88x921755=45 MHz. If this 45 MHz signal is mixed with the first transmit signal F1 of 1800 MHz, there will also be 1800+45=1845 MHz, which is the frequency of the third-order IM product. If this signal is in anti-phase and has equal amplitude to the unwanted third-order IM product, a reduction of the unwanted signal is possible.
In the case of a communications system, the difference between the transmitting frequency and the receiving frequency is the channel spacing, while a similar effect may be obtained if the second-order IM product used for compensation lies at a frequency equal to the channel spacing. In the above example the transmitting frequency F1 is, for example, 1800 MHz, the second input signal F2, for example, 1755 MHz and the receiving frequency of a nearby receiver, for example, 1845 MHz. The amplified third-order IM product, also at 1845 MHz, is radiated by the antenna and may reduce the receiving sensitivity of a nearby receiver. With the aid of the same measures as with the signal processing system, the third-order IM product may be reduced, so that a reduction of the receiving sensitivity of a nearby receiver is avoided.
In a first embodiment of the signal processing system according to the invention, two signals F1 and F2 are present on the input of the two-port. The resulting second-order IM product F1xe2x88x92F2 on the output of the two-port is frequency-selectively applied with the correct phase and amplitude from the two-port output to the two-port input through the external network, at which input the product is mixed with the transmit signal F1 by means of the non-linear amplification of the two-port. The resulting new mixed product F1+(F1xe2x88x92F2)=2*F1xe2x88x92F2 has, when the external network is suitably selected, exactly the same amplitude, but an opposite phase to the unwanted third-order IM product 2*F1xe2x88x92F2. When the two signals are added together, the unwanted third-order IM product is compensated for. Since the external network is frequency-selective, mainly the second-order IM product is present on the input of the two-port, whereas the original input signals are fed back the least possible from the output to the input.
In a further embodiment of the signal processing system according to the invention, in which the two-port comprises an internal feedback, the input signal F2 is not present on the input of the two-port, but on the output of the two-port. This signal F2 gives rise to backward intermodulation because, as a result of the internal feedback, it reaches the input where the signal F1 is already available. As a result of intermodulation of the signals F2 and F1, due to the non-linear amplification, unwanted intermodulation products arise. From this point on, a similar compensation to that described in the first embodiment is possible.
In a further embodiment of the signal processing system according to the invention, in which the external network has a frequency-dependent impedance between the output of the two-port and the input of the two-port, this frequency-dependent impedance forms a lower impedance for the second-order IM product F1xe2x88x92F2 than for the signals F1 and F2. This causes the feedback of F1xe2x88x92F2 to be stronger than for F1 and F2, so that the amplification of F1 and F2 is affected only minimally. The second-order IM product F1xe2x88x92F2 is then used in similar fashion to that of the first embodiment to compensate for the third-order IM product 2*F1xe2x88x92F2 by means of intermodulation, phase shifting and amplitude matching.
In a further embodiment of the signal processing system according to the invention, in which the external network has a frequency-dependent impedance between the input of the two-port and a terminal having reference potential, the frequency selectivity of the feedback is determined by the frequency-dependent impedance. As this impedance forms a higher impedance for the second-order IM product F1xe2x88x92F2 than for the signals having the frequencies F1 and F2, the signals having the frequencies F1 and F2 in the feedback signal are attenuated more than the second-order IM product F1xe2x88x92F2. As a result, the amplification of F1 and F2 is affected minimally, whereas the second-order IM product is still available on the input and can be used as in the first embodiment to compensate for the third-order IM product. The external network as a whole provides a suitable phase and amplitude of the second-order IM product.
In a further embodiment of the signal processing system according to the invention, in which the two-port as described in the previous embodiment also includes a bipolar transistor, the frequency-selective impedance is used in a different manner. Again the second-order IM product F1xe2x88x92F2 is used to compensate for the third-order IM product 2*F1xe2x88x92F2. At the frequency of the second-order IM product the frequency-dependent impedance connected between the input of the two-port and a reference potential has a lower impedance than at the transmitting frequency F1 and also a lower impedance than the output impedance of the output circuit. This makes for increased amplitude of the base current of the second-order IM product F1xe2x88x92F2 relative to the other signal components present. The resulting second-order IM product, after being amplified by the bipolar transistor, is fed back by the external network from the output to the input of the two-port together with the other signals present in the two-port. As the second-order IM product in this feedback signal is amplified relative to the other signals present, in effect a frequency-selective feedback is obtained. Since the feedback, together with the base-connected series LC circuit, provides that the second-order IM product F1xe2x88x92F2 having the right phase and amplitude is available on the base, it is possible with the resulting mixing product of the second-order IM product F2xe2x88x92F1 and the send signal F1 to compensate for the third-order IM signal component 2*F1xe2x88x92F2.